This invention is directed to a corrosion resistant ferrous article comprising a ferrous substrate having an alloy coating on a surface thereof, which coating consists essentially of aluminum, zinc, magnesium and silicon and to a method of producing such article.
Galvanizing is the conventional way of providing a ferrous article with a corrosion resistant coating. Although zinc alone provides substantial protection against corrosion and is acceptable for many purposes, its corrosion resistant properties can be improved by adding minor percentages of other metals to the molten bath of zinc in which the ferrous articles are dipped. Examples thereof are:
U.S. Pat. No. 3,320,040, directed to the production of a paintable, spangle-free galvanized ferrous article having a ductile, adherent coating comprising 0.1-3.5% dissolved Al to regulate the thickness of the intermetallic layer between the coating and the ferrous substrate, 0.02-0.2% Mg to improve adherence of paint to the coating, less than 0.1% Pb to provide a substantially spangle-free surface, and the balance Zn;
U.S. Pat. No. 3,993,482, directed to a zinc base alloy coating for a ferrous substrate. The coating contains up to about 2% aluminum plus larger amounts of magnesium and a trace of chromium, with an optimum coating alloy comprising 5% Mg, 2% Al, and balance Zn, except for 0.2% Cr to increase corrosion resistance;
U.S. Pat. No. 3,505,042, directed to a hot-dip coating method, wherein a ferrous metal article is coated with a zinc alloy containing 1.5-5% by weight magnesium, plus 0.15-0.5% aluminum to retard iron-zinc alloy growth in the intermetallic layer and reduce surface oxidation losses in the bath;
British Pat. No. 1,125,965, cited in the specification of U.S. Pat. No. 3,993,482, is briefly reviewed by the U.S. patentee as teaching a zinc-based alloy for use against widespread corrosion. The alloy contains 1-4% Mg and 0.05 to 5% Al, with best results being obtained with about 2.5% Mg and about 4.4% Al.
In situations where high resistance to an SO.sub.2 environment is desired, ferrous articles are coated with aluminum containing lesser amounts of other metals. Examples of such coatings are described in:
U.S. Pat. No. 2,406,245, directed to a process for hot-dip coating ferrous articles in a molten Al bath containing 7.5-9.5% Si to improve adhesion and 0.02-2.5% Mg, to improve luster and spangle;
U.S. Pat. No. 3,010,190, directed to an aluminum alloy coating for a ferrous metal base, which coating contains up to 6% Si to decrease the intermetallic layer and 0.5-2% Zn to eliminate the surface roughening effect and steel grey appearance imparted to the coating by the Si; and
U.S. Pat. No. 3,026,606, directed to hot-dip aluminum coating of ferrous articles in an aluminum bath containing Mg and Si in a stoichiometrical relationship to yield up to 25% Mg.sub.2 Si in the coating.
Whereas zinc coatings on steel offer galvanic protection to unprotected edges of the steel base in salt water, such coatings have relatively poor resistance to SO.sub.2 environments. Aluminum coatings on steel are noted for their durability, but fail to provide galvanic protection to unprotected edges of the steel base. Therefore, combinations of the two metals, zinc and aluminum, have been employed to produce coatings having the desirable features of both types of coatings. Examples of aluminum-zinc coatings are described in:
U.S. Pat. No. 3,505,043, directed to a metallic coated ferrous product having a eutectic type aluminum-zinc coating containing, by weight, 3-17% aluminum, 1-5% magnesium, balance zinc; and
German Patent Application No. 2,146,376 which discloses that if steel is first coated with pure zinc, it can then be coated with a zinc alloy which contains up to 50% Al, preferably no more than 35% Al. A specific example of the coating is one having 20% Al, 5% Mg, 1% Si, balance Zn;
U.S. Pat. Nos. 3,343,930 and 3,393,089, directed, respectively, to the product and to the method of hot-dip coating a ferrous base with an alloy comprising, by weight, 25 to 70% aluminum, and the balance essentially zinc. A small amount, e.g., about 1.6% of silicon is included to insure adherence of the coating to the substrate. A typical commercial sheet product, which the inventors of this invention believe represent the optimum in corrosion resistance performance for commercial hot-dip coated products, consists of a steel base having a coating comprised of an alloy overlay and a thin intermetallic layer intermediate said overlay and steel base, and a composition of, by weight, 55% aluminum, 1.6% silicon, balance essentially Zn (hereinafter referred to as 55Al-Zn). The as-cast microstructure of the overlay has cored aluminum-rich dendrites and zinc-rich interdendritic constituents.
In U.S. Pat. No. 3,505,043, up to 17% Al is added to the Zn coating along with 1.5% to 4% Mg. In German Patent Application No. 2,146,376, a typical alloy coating contains 20% Al, 5% Mg, 1% Si, balance Zn. With each such zinc-base alloy coating the corrosion resistance performance would be from 11/2 to 3 times worse than 55Al-Zn, particularly in the coating's resistance to SO.sub.2 environments.
Surprisingly, it was discovered with the present invention that with an aluminum/zinc alloy coating containing sufficient amounts of both Si and Mg, a product results which has even better overall corrosion resistant properties and good appearance compared to the 55Al-Zn product. Such discovery will become apparent from the description which follows.